1. Field of the Invention
This invention relates to a catalytic process for effecting the benzylic oxidation of toluene.
2. Description of the Prior Art
It has hitherto been proposed to prepare benzylic esters by the acyloxylation of an appropriate aromatic hydrocarbon either in the vapor or liquid phase. Since the underlying reaction involves a benzylic oxidation mechanism, a catalyst is required for carrying out the process. The most effective catalyst for this purpose reported in the prior art is palladium in the form of a palladous compound. It has been further proposed to utilize the palladium catalyst in combination with another compound for the purported purpose of enhancing the activity thereof.
U.S. Pat. No. 3,547,982 is particularly representative of the prior art directed to a liquid phase acyloxylation proces in which the palladium catalyst is modified with a variety of other compounds soluble in the reaction mixture, notably various compounds of tin or phosphorus. More recently in U.S. Ser. No. 029,819, filed Apr. 13, 1979, now abandoned, a co-catalyst system comprised of Pd(II) and Pb(II) values is taught as being especially effective in catalyzing the liquid phase acyloxylation of a variety of compounds having in common an allylic residue.
While the resultant product of the acyloxylation reaction is an ester, the latter only represents a precursor of that which is normally desired; namely, the corresponding alcohol. Consequently, the carboxylic acid reactant essentially amounts to a recycle stream in the process. From the standpoint of convenience, raw material cost and effectiveness, the lower fatty acids and, specifically acetic acid, have been considered as representing the most suitable carboxylic acids for carrying out the process. However, the vapors of the lower fatty acids are extremely corrosive and the use thereof is desirably to be avoided, particularly in any large scale commercial operation considering the substantial amount of recycling involved.
The higher fatty acids as well as a variety of aromatic and acyclic diacids, while substantially noncorrosive in nature, suffer on the other hand, for lacking mutual solubility with the benzylic substrate. In liquid phase homogeneous catalysis reactions in systems where there are a plurality of reactants, it is essential to have the responsive reactants mutually soluble or compatible with one another in order to conduct the reaction in an effective manner. The aforementioned U.S. Ser. No. 029,819, beyond teaching a novel co-catalyst system for the acyloxylation reaction, represents the only known art which addresses the problem of employing a higher fatty acid or applicable diacid in a reaction of this type. Therein it is taught that the requisite homogeneity can be obtained through the use of a coupling solvent in the reaction mixture. Although effective for the purpose indicated, a coupling solvent adds a further cost to the process and undesirably necessitates yet another recycle stream.